What you need to do is, your sister just need to remove any external application plugin from her profile like, it could be done by clicking on to the apps from account settings and remove one of those suspicious application from it, and then if she will be doing re-login, then I am sure she might have her account on the consistent stage, try it and let me know if still she having problems while get rid of this!!

Please click the link that i have provided to you below in order to help secure your account from this